In line with rapid increase in use of fossil fuels, demand for alternative energy or clean energy is increasing. Thus, the field of power generation and electricity storage that use electrochemical reaction is most actively studied.
As a representative example of electrochemical devices using electrochemical energy, secondary batteries are currently used and use thereof is gradually expanding.
Recently, as technology for portable devices, such as portable computers, portable phones, cameras, and the like, continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, research on lithium secondary batteries having high energy density, high operating potential, long cycle lifespan and low self-discharge rate has been underway and such lithium secondary batteries are commercially available and widely used.
In addition, as interest in environmental problems is increasing, research into electric vehicles, hybrid electric vehicles, and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is underway. As a power source of electric vehicles, hybrid electric vehicles, and the like, nickel-metal hydride secondary batteries are mainly used. However, research into lithium secondary batteries having high energy density and high discharge voltage is actively carried out and some of the lithium secondary batteries are commercially available.
Existing typical lithium secondary batteries use graphite as a negative electrode active material, and are charged and discharged while repeating the process that the lithium ions of a positive electrode are insert into and released from a negative electrode. Theoretical capacities of batteries depend on electrode active material types but charge and discharge capacity is mostly reduced as cycle life progresses.
A main cause of such a phenomenon is insufficient function of an active material, due to separation between electrode active materials or between an electrode active material and a collector due to volume change of an electrode, occurred according to repetitive charge and discharge of a battery. In addition, during insertion and release processes, lithium ions inserted into a negative electrode are not normally released therefrom and, thus, active sites of the negative electrode reduces, thereby deteriorating charge/discharge capacity and battery lifespan.
In particular, when, in order to increase discharge capacity, natural graphite having a theoretical discharge capacity of 372 mAh/g is used with a material such as silicon, tin, silicon-tin alloy, or the like having a large discharge capacity, volume of a material dramatically expands according to progression of charge and discharge and, as such, a negative electrode material is separated from an electrode material. As a result, repetitive cycles are progressed and, thus, capacity of a battery is dramatically reduced.
Thus, there is an urgent need in the art to study a binder and an electrode material that may have strong adhesive strength so as to prevent separation between electrode active material components or separation between an electrode active material and a current collector and may achieve structural stability of an electrode by controlling volume expansion of an electrode active material caused as charging and discharging are repeated and, accordingly, enhance battery performance.
A conventional solvent-based binder, i.e., polyvinylidene fluoride (PVdF), does not satisfy meet such requirements and thus, recently, a method of using binders prepared by preparing emulsion particles by aqueous polymerization of styrene-butadiene rubber (SBR) and mixing the emulsion particles with a neutralizing agent and the like has been proposed and is currently commercially available. These binders are eco-friendly and used in a small amount and thus may increase battery capacity.
Therefore, there is an urgent need to develop a binder that enhances cycle characteristics of a battery, imparts structural stability to an electrode, and has high adhesive strength.